The question was surely running through the minds of Manchester City fans after Southampton were gifted each of their three goals and, once again, at full-time: when does a lead become insurmountable rather than merely mammoth?

The answer could well lie in the Premier League table at around 6pm on Sunday.

Should Manchester United overcome Everton at Old Trafford, they will surge 12 points clear at the top and leave Roberto Mancini’s team praying for their neighbours to do the splits with the finishing line in sight.

With three months of the campaign to run, it seems absurdly early for the Devon Loch references to begin.

But, following City’s desperate display at St Mary’s Stadium, Sir Alex Ferguson’s team have already taken on the mantle of run-away leaders.

Even Mancini admitted: “I think now we have a 10 per cent chance, maybe. I don’t think that we’ll recover 12 points; it will be really difficult.”

Those of us who sneer at the Spanish and Scottish leagues for harbouring runaway leaders will be squirming should the English top flight’s two-horse title race turn into a single-thoroughbred procession.

United supporters will be telling themselves not to get too giddy following their title collapse last season, that began with the failure to hold on to a 4-2 lead against Everton in April.

They will point out the dual distractions of the FA Cup and a Champions League knockout campaign starting with Tuesday’s glamour tie with Real Madrid, while also bearing in mind the club’s habit of stockpiling bodies in the physio’s room in the closing months of the season.

But Sir Alex’s biggest domestic challenge in the weeks ahead will be to keep the feet of his players’ firmly on the ground.

Following a solid January in which they did not concede a single goal, City have taken only two points out of a possible nine.

After being shut out by QPR and out-played by Liverpool, a 3-1 shellacking to a team fighting relegation and who are just getting used to a new manager felt altogether more serious.

It was a horror show for City, from the moment Barry lost possession in the build-up to Jason Puncheon’s early opener to when they were finally put out of their misery at the final whistle, prompting an out-pouring of joy from the elated home support.

The highlights reel will blush when replays of Joe Hart’s disastrous attempt to gather Rickie Lambert’s arrow-straight shot and Barry’s calamitous own-goal are screened over and over again.

But the biggest worry for Mancini and the City top brass should not merely be the team’s gift for making things difficult for themselves, but the absence of any semblance of a late comeback.

Over the last two seasons, City have re-affirmed the old truth that great teams love adversity.

But where was the street-fighter that is the hallmark of real champions at St Mary’s? Where was the team who eventually grind down their opponents? Where was the spirit?

It said a lot that Southampton were able to close out the game with relative comfort. Mauricio Pocchetino's team deserve great credit for their relentless pressing and the adventure of their play, but champions are not supposed to be swatted aside so easily.

The malaise evident in City’s disastrous Champions League campaign has spread to domestic engagements. As the weeks go by, it seems more chronic.

Nearly everywhere you looked at St Mary’s was a player stinking out a sky blue shirt. Only perhaps the commendable Pablo Zabaleta could walk off the field safe in the knowledge that he had not let the team down.

City fielded a strong starting eleven, even if the absence of Carlos Tevez for personal reasons meant that there was not a recognised forward among the substitutes.

But the nerves initially showed by stand-in centre-back Javi Garcia swiftly spread to his team-mates, and eventually proved terminal to City’s chances of getting three points.

The ruthlessness has been misplaced. So has the authority. It can’t just be blamed on Vincent Kompany wearing a smart suit in the stands rather than his playing kit.